Uni-, bi-, and tri-directional wetting caused by nanostructures with anisotropic surface energies.

Wetting is a pervasive phenomenon that governs many natural and artificial processes. Asymmetric wetting along a single axis, in particular, has generated considerable interest but has thus far been achieved only by the creation of structural anisotropy. In this paper, we report that such directional wetting can also be achieved by anisotropically coating nanostructure surfaces with materials that modify the nanostructure surface energy, a phenomenon that has not been observed in natural or artificial systems thus far. Moreover, by combining this newfound chemical influence on wetting with topographic features, we are able to restrict wetting in one, two and three directions. A model that explains these findings in terms of anisotropy of the pinning forces at the triple phase contact line is presented. Through the resulting insights, a flexible method for precise control of wetting is created.